Gas-filled discharge tubes are extensively used as protection against transient voltages in electronic equipment of various kinds, e.g. telephone equipment, computers, and safety systems. Discharge tubes for this special purpose consist, as a rule, of at least two electrodes which, with a suitable distance between them, are joined to an insulating body, so as to form at least one discharge gap in a discharge chamber that is vacuum-tight at normal temperatures and which encloses a gas of a suitable kind at a suitable pressure. The insulating body is, as a rule, made of ceramics. Two electrode tubes are used most often. The tube can be connected between conducting points which can be exposed to transient voltages, or between such a point and the ground. Three-electrode tubes are also being used. A central electrode, as a rule, is connected to the ground, whereas the outer electrodes are connected to the points to be protected.
The electrode material of the tube, the size of the electrode gap and the type and pressure of the gas will determine the firing or striking voltage of the tube. The latter must be adjusted in such a way that the tube will not ignite at the normally applied voltages. However, if voltages originate which could harm the equipment, the tube will ignite and cause the voltage to drop in the tube and thus in the protected equipment, thereby preventing the occurrence of damage.
Since the discharges occuring between the electrodes generate a displacement of materials between same, there is a certain risk of disturbances in the form e.g. of short circuits in the electrode gap. For that reason, gaps between the electrodes are rarely very small. The gaps mostly used are of the order of magnitude of 0.4-0.5 mm. As filler gaps, argon is frequently used, possibly with an addition of about 11% of hydrogen. Sometimes krypton and xenon are used instead of argon. In connection with copper electrodes and an electrode gap of about 0.5 mm, a gas pressure is frequently selected which measures about 10 kPa at normal room temperature. This will ensure a frequently desirable starting voltage of about 350 V. If, for any reason, it is desired to manufacture a tube for higher gas pressures, the electrode gap should be reduced accordingly, since the product of gap and pressure must not change if the firing voltage is to remain unchanged as well. As already stated above, gap disturbances can be a problem.
As mentioned before, an insulating body of ceramics, designed as a hollow cylinder, is used most frequently. The tube is usually manufactured in such a way that the end surfaces of the ceramic unit are metalized, often by means of a coat of molybdenum-manganese and an overcoat of nickel. The electrodes can then be soldered to this metalized layer. If the electrodes are of copper, e.g. a silver-copper eutectic can be used as suitable soldering material at about 800.degree. C. Other electrode material may require a different kind of soldering metals. As a rule, the soldering, together with the rest of the process, follows either one of the two methods described below.
In one case a ring of soldering material is positioned on the portion of the electrode surface to be soldered to one metalized end surface of the ceramic tube. The ceramic tube is placed on the solder ring, while a new solder ring is positioned on the other metalized end surface of the ceramic tube, and the second electrode is placed on that other end. This electrode has been outfitted with a passage-providing narrow copper tube, to form an open channel for the internal volume of the tube. The soldering is done most often in a belt furnace with reducing gas, usually hydrogen or a mixture of hydrogen and nitrogen. The temperature depends on the soldering material, with the silver-copper eutectic, this is, as mentioned, about 800.degree. C. The soldering is followed by vacuum pumping at about 400.degree. C., and by a refilling or replenishing with the desired gas up to the desired pressure. Pumping and refilling or replenishing are frequently performed manually in so-called pump boxes. However, semi-automatic devices are occasionally also used. Vacuum pumping and refilling or replenishing occur through the tube, the so-called exhaust tube, which one of the electrodes has been provided with. At times, a small portion of the refilling or replenishing gas is replaced by tritium, a radio-active isotope of hydrogen, which has a certain stabilizing effect on the firing voltage of the tube. After vacuum pumping and gas refilling or replenishing have been completed, the copper tube is nipped off near the electrode. This nipping off, as a result of cold diffusion, will cause a vacuum-tight joint. The manufacturing process concludes with an electrical stabilizing treatment prior to the final test.
The second method calls for stacking of the tube parts in the same manner as above. However, in this case no exhaust tube is used. The stacked tube parts are placed in suitable numbers on a plate of suitable material, and a number of these plates are placed, jointly, into a furnace. The furnace is pumped down to a vacuum of about 0.01 Pa at a temperature slightly below the melting point of the soldering material. Since the tube parts are stacked loosely, there will also be a vacuum in the internal volume of the stacks. Subsequently, at the same temperature, replenishing gas is fed into the furnace and thus also into the stacked tubes. The temperature is then raised, and thus, the tubes are soldered together within the gaseous atmosphere. As in the previous case, after cooling, an electrical stabilizing treatment is applied prior to the final test.
Also in the case of tubes manufactured according to the second method, it is sometimes desired to mix a small amount of tritium with the filler gas. However, since the gas in the furnace is, after cooling, ventilated after each pumping turn or cycle, it is inconceivable to mix tritium with the filler gas. Instead, a diffusion process, cf. e.g. Swedish Pat. No. 375,201 is frequently used after the completed pumping process.
Variants of these two manufacturing methods are sometimes used, but the feature that all of them have in common is that they are very cumbersome, require a lot of energy and manufacturing equipment, and are not suited for automatic in-line production.